Surface treated pigments have been used to improve the wetting and dispersion of pigments and fillers in inks, coatings, resins and cosmetics. Passivation of the pigment and filler surfaces to reduce chemical interaction with the vehicle is another application of surface modifications. In cosmetics, surface coatings of pigments and fillers offer the added benefits of improvements in skin feel, easier spreading and blending on the skin, reduction in irritation due to mechanical abrasion and reduced drying of the skin from oil and moisture absorption.
Types of surface coatings used in cosmetic applications have included fatty acids, lecithin, mineral waxes, e.g. polyethylene, vegetable waxes, starches, peptides, polysaccharides, acyl amino acids, titanate esters, fluorophosphates, silicones and silanes. The modification of substrates with silanes is well-known in the art and is described by Arkles in Chemtech, 7(12), 766, (1977), which is herein incorporated by reference. Silane coupling agents and reactive silicones are particularly useful surface treatments for use in dispersed systems due to the formation of chemical bonds between the treating compound and the pigment surface that prevents solubilization of the coating during processing of the finished product. Silane and silicone surface treated pigments have been used in a variety of cosmetic formulations, including foundation, mascara, eye liner, eye shadow, lip color and blush, in which the powder is dispersed in a liquid phase. The most common hydrophilic silane utilized is PEG6-9-silane (methoxypoly(ethyleneoxy)6-9propyltrimethoxysilane). However, PEG6-9-silane can affect the formation of emulsions, causing excessive pigment flotation. Further, the long-term oxidative stability of PEG6-9-silane and degradation products of ethyleneoxide derived materials have potential health effects that may be of concern in some formulations.
The utilization of modified amino acids as surface treatments for particulates, including pigments and fillers, is well-known in cosmetic and personal care technology. Examples are coated pigments and fillers with excellent skin feel and reduced potential for skin abrasion, prepared by utilizing salts of acyl amino acids such as aluminum N-myristoyl-L-glutamate (see U.S. Pat. No. 4,606,914 of Miyoshi); platy pigments with improved tactile properties, prepared by precipitation of acylamino acids such as N-lauroyllysine on the surface of talcs (see, for example, U.S. Pat. No. 5,326,392 of Miller); and skin treatments prepared by treating pigments and fillers with combinations of N-acylamino acids, N-acylamino acid salts and fatty acids (see U.S. Pat. No. 7,374,783 of Hasegawa). All of these coatings are produced by adsorbing or precipitating amino acids on particles. For the most part, these coatings are relatively hydrophobic since they are derived from N-acyl substituted amino acids in which the acyl group has six or more carbons, most often lauroyl (12 carbons). The use of particles treated with these systems is restricted to oil based color cosmetics, either anhydrous formulations or water-in-oil emulsions, because particles with adsorbed hydrophobic amino acids will not disperse in the continuous aqueous phase. The pigment phase of the finished emulsion must be dispersed in the external phase of the emulsion to allow the mass tone of the product to be similar to that achieved on the skin after application.
There are many instances in which the benefits of amino acid modified particles would be desirable in oil-in-water formulations. However, amino acids simply adsorbed onto particulates tend to destabilize oil-in-water emulsions. They also tend to coalesce dispersed oil by physically bridging on the surface of the particle by adsorption phenomena. Further, particles with adsorbed hydrophilic amino acids tend to be intrinsically unstable at water-oil interfaces since the amino acid tends to desorb from the particle, at once changing the surface characteristics of the particle and changing the aqueous environment by introducing soluble amino acids which tend to be strong zwitter-ions.
While the immobilization of enzymes and amino acids has been disclosed by Weetall (U.S. Pat. No. 3,652,761), these systems were designed for fixed-bed catalysis, and the particle dimensions are not suitable for the purpose of forming stable dispersions.